For a fraction of lensing events, light curves exhibit deviations from the 1L1S form. Such
deviations are most commonly caused by the duality of the lens [1]} or the source
[2]}. Hereafter, we refer to the three-object events with binary lens or binary
source stars as 2L1S or 1L2S events, respectively. At the time when such anomalous events were
first found, for example, MACHO LMC 1 [3]} and OGLE 7 [4]} 2L1S
events and MACHO LMC 96-2 [5]} 1L2S event, interpreting the observed lensing light
curves was a challenging task due to the difficulty of modeling caused by various technical issues.
The first of these issues was the increased number of lensing parameters with the addition of the
extra lens or source component, and this made it difficult to find a lensing solution, that is,
a set of lensing parameters that best explain the observed light curves, via a grid approach.
Second, the \(\chi ^2\)  surface in the parameter plane was complex, especially for 2L1S events, due
to the discontinuity in lensing magnifications caused by the formation of caustics, and this made
it difficult to find a solution via a simple \(\chi ^2\)  minimization approach. Third, computing
finite-source magnifications during the source star's crossing over the caustic required heavy
computations, and this hampered prompt interpretations of anomalous lensing events. The majority
of these difficulties in the interpretation of multiple-object lensing events has been resolved
with the adoption of sophisticated logic that is optimized for finding lensing solutions in a
complex parameter space, such as the Markov Chain Monte Carlo (MCMC) method, together with the
development of efficient methods for computing finite-source magnifications, such as the map-making
method, for example, [6]}, the adaptive ray-shooting method, for example, [7]},
and the image contour method, for example, [8]} and [9]}. As a result,
three-object modelings are routinely being conducted for most anomalous events.
